The Sciences

One Particle, Two Worlds

The transition from quantum to classical. 

By Susan KruglinskiJun 27, 2004 5:00 AM


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In some ways, the world seems divided in two: our classical world, where objects have well-defined locations, and the quantum realm, where particles seem to be everywhere at once. University of Vienna physicists designed an experiment to traverse these worlds with a change in temperature.

Using a laser, Anton Zeilinger, Markus Arndt, and their team heated giant carbon molecules to more than 5,000 degrees Fahrenheit in an airless environment. Above that temperature the molecules acted in a classical way. But as the temperature dropped, they switched into a wavelike state in which their location could be described only in the statistical terms of quantum physics—they no longer seemed to be in any one place. This transition may depend not on temperature but on the particle’s relationship to its surroundings. When the carbon molecule was hot, it emitted radiation that interacted with nearby walls, giving it a definite location. When cooled, the molecule stopped radiating and became an isolated quantum-style object.

The transition from quantum to classical, called decoherence, has never before been demonstrated by using heat. The work could have a big payoff. Researchers worldwide are attempting to control decoherence to build a quantum computer that, if perfected, could lead to unbreakable encryption and ultrafast information processing.

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